U.S. patent number 5,539,645 [Application Number 08/155,060] was granted by the patent office on 1996-07-23 for traffic monitoring system with reduced communications requirements.
This patent grant is currently assigned to Philips Electronics North America Corporation. Invention is credited to Indur B. Mandhyan, Karen I. Trovato.
United States Patent |
5,539,645 |
Mandhyan , et al. |
July 23, 1996 |
Traffic monitoring system with reduced communications
requirements
Abstract
Monitoring of traffic on selected routes requires little
communication time, through reporting only instances of abnormal
speed. During a calibration phase calibrant vehicles are operated
along the selected routes with sufficient frequency and for enough
days to provide meaningful data. Each calibrant vehicle carries a
differential GPS receiver for measuring location accurately.
Average speeds for intervals of, for example, 15 seconds, are
stored, with the time and place of observation. The data from all
calibrant vehicles are then analyzed to determine patterns of mean
speed and bandwidth. In the monitoring phase probe vehicles are
deployed, each carrying similar GPS, a computer in which the
patterns are stored, and a radio for automatically reporting speeds
which are out of bandwidth for that time and place.
Inventors: |
Mandhyan; Indur B.
(Croton-on-Hudson, NY), Trovato; Karen I. (Putnam Valley,
NY) |
Assignee: |
Philips Electronics North America
Corporation (New York, NY)
|
Family
ID: |
22553970 |
Appl.
No.: |
08/155,060 |
Filed: |
November 19, 1993 |
Current U.S.
Class: |
701/119; 340/905;
701/117 |
Current CPC
Class: |
G08G
1/0104 (20130101) |
Current International
Class: |
G08G
1/01 (20060101); G08G 001/09 (); G06G 007/76 () |
Field of
Search: |
;364/436,424.02,461,438
;340/905,995,996 ;348/149 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Teska; Kevin J.
Assistant Examiner: Phan; Thai
Attorney, Agent or Firm: Treacy; David R.
Claims
What is claimed is:
1. A method of estimating quantitive data describing the flow of
traffic, comprising the steps of:
a) providing a plurality of calibrant vehicles,
b) providing each calibrant vehicle with respective means for
acquiring data from which speed of the calibrant vehicle at
different times and locations can be determined; and for
transmitting the acquired data to a receiving station,
c) providing at least one receiving station having means for
receiving said data transmitted by respective calibrant
vehicles,
d) at spaced times approximately equal to predetermined times of a
respective day, dispatching a respective calibrant vehicle for
operation over a substantially predetermined route,
e) during at least the portion of the day that each respective
vehicle is being operated over said route, controlling said
respective vehicle to record said data,
f) transmitting the recorded data to said at least one receiving
station,
g) computing subsegment speed samples for each calibrant vehicle
from which said data have been received, and determining baseline
data having a time-varying bandwidth descriptive of traffic
conditions on respective segments of said route for at least one
combination of time of day and traffic conditions,
h) analyzing said data received from said calibrant vehicles to
determine the relationship between the number of said calibrant
vehicles and the reliability of traffic flow estimation based
thereon, and selecting a first number of probe vehicles whose
reporting will provide a given reliability of traffic flow
estimation,
i) then deploying said number of probe vehicles at least one time
of day and traffic conditions corresponding to said at least one
combination, each probe vehicle having respective means for
acquiring data from which subsegment information including the
speed of that probe vehicle at different times and locations can be
determined,
j) in response to predetermined criteria, controlling at least one
of said probe vehicles to transmit said subsegment information,
and
k) computing estimated traffic flow along at least one segment of
said route based at least partly on the transmitted subsegment
information.
2. A method as claimed in claim 1, characterized in that step i)
comprises providing each probe vehicle with means for determining
the location of the respective vehicle; causing each probe vehicle
to determine its location at respective instants of time separated
by intervals of approximately a given period of time, recording
probe data corresponding to the determined location and the
corresponding instant of time, and determining and recording
subsegment information based at least in part on said probe
data.
3. A method as claimed in claim 2, characterized in that each probe
vehicle comprises a respective radio transmitter,
the step of controlling at least one of said probe vehicles
comprises controlling the respective radio transmitter to transmit
the respective subsegment information in a respective time slot
over a radio channel, and
said subsegment information is stored in said one of said probes no
later than the next occurring respective time slot for that probe
in which transmission is successful.
4. A method as claimed in claim 3, characterized in that a
plurality of receiving stations are provided, having overlapping
operational ranges, each receiving station including means for
transmitting control and confirmation signals,
in response to said predetermined criteria, said one of said probe
vehicles transmits said subsegment information,
upon receipt of a confirmation signal from a receiving station, the
probe repeats the step of determining its location, recording probe
data, and determining and recording subsegment information, and
upon failure to receive a confirmation signal, the probe transmits
said subsegment information during the next occurring respective
time slot.
5. A method as claimed in claim 1, wherein a multiplicity of probe
vehicles are provided, each probe vehicle being operated at the
discretion of the respective vehicle operator, further comprising
the steps of
transmitting an identification signal from a given probe vehicle
when it is placed into operation on a said route,
upon receipt of said identification signal by said one receiving
station, determining if said given probe vehicle is within
operational range,
determining if the number of probe vehicles already communicating
on routes within operational range of said one receiving station is
less than said first number, and
upon determination that said number of probe vehicles already
communicating is less than said first number, transmitting control
signals to said given probe vehicle to cause at least one further
transmission from said given probe vehicle.
6. A method as claimed in claim 1, characterized in that step b)
comprises providing each calibrant vehicle with respective means
for determining the location of the respective vehicle at
respective instants of time separated by intervals of approximately
a given period of time, for determining the time of each said
respective instant, and for recording data corresponding to the
determined location and said time for each respective instant; and
respective means for transmitting the recorded data.
7. A method as claimed in claim 6, characterized in that each
calibrant vehicle records and stores data for each of said instants
of time while being operated over at least a segment of the entire
predetermined route, prior to transmitting the stored data to said
receiving station.
8. A method as claimed in claim 6, characterized in that each
calibrant vehicle records and stores data for each of said instants
of time while being operated over the entire predetermined route,
prior to transmitting the stored data to said receiving
station.
9. A method of estimating quantitive data describing the flow of
traffic, comprising the steps of:
a) providing a plurality of calibrant vehicles,
b) providing each calibrant vehicle with respective means for
determining the location of the respective vehicle at respective
instants of time separated by intervals of approximately a given
period of time, for determining the time of each said respective
instant, and for recording data corresponding to the determined
location and said time for each respective instant; and respective
means for transmitting the recorded data,
c) providing at least one receiving station having means for
receiving said data transmitted by respective calibrant
vehicles,
d) at spaced times approximately equal to predetermined times of a
respective day, dispatching a respective calibrant vehicle for
operation over a substantially predetermined route,
e) during at least the portion of the day that each respective
vehicle is being operated over said route, controlling said
respective vehicle to record said data,
f) transmitting the recorded data to said at least one receiving
station,
g) computing subsegment speed samples for each calibrant vehicle
from which said data have been received, and determining baseline
data having a time-varying bandwidth descriptive of traffic
conditions on respective segments of said route for at least one
combination of time of day and traffic conditions,
h) analyzing said data received from said calibrant vehicles to
determine the relationship between the number of said calibrant
vehicles and the reliability of traffic flow estimation based
thereon, and selecting a first number of probe vehicles, less than
the number of said plurality of calibrant vehicles, whose reporting
will provide a given reliability of traffic flow estimation,
i) then deploying a second number of probe vehicles at least one
time of day and traffic conditions corresponding to said at least
one combination, each deployed probe vehicle having respective
means for determining the location of the respective vehicle at
respective instants of time separated by intervals of approximately
a given period of time, for determining the time of each said
respective instant, for computing average subsegment speed between
the most recent determination of location and the previous
determination for that probe vehicle, and for comparing said
average subsegment speed with said baseline data having a
time-varying bandwidth descriptive of traffic conditions on the
segments of said route in which the latest location lies, and for
determining whether that average subsegment speed is a normal value
falling within said bandwidth for the combination of time of day,
segment and traffic conditions,
j) responsive to determination that a given probe vehicle's
subsegment speed is an abnormal speed not falling within said
bandwidth, controlling said means for transmitting in said given
probe vehicle to transmit information related to the computed
subsegment speed, and
k) computing estimated traffic flow along at least one segment of
said route based at least in part on the transmitted
information.
10. A method as claimed in claim 9, further comprising
controlling each of said second number of probe vehicles to
transmit a request for recognition automatically when the
respective probe vehicle is put into an operating mode on said
route,
providing at least one receiving station having means for receiving
transmissions from respective probe vehicles,
upon receipt of said request for recognition by said at least one
receiving station, determining whether a number of probe vehicles
equal at least to said second number have transmitted requests for
recognition, and
responsive to the number of probe vehicles requesting recognition
exceeding said second number, transmitting a control message not to
transmit further information.
11. A method as claimed in claim 9, further comprising
controlling each of said second number of probe vehicles to
transmit a request for recognition automatically when the
respective probe vehicle is put into an operating mode on said
route,
providing at least one receiving station having means for receiving
transmissions from respective probe vehicles,
counting the number of said requests for recognition received by
the receiving stations, and comparing the counted number to said
second number, and
responsive to the counted number being less than said second
number, providing an alert indication to a system operator.
12. A method as claimed in claim 1, further comprising:
storing in said probe vehicle at least one bandwidth determined for
a given segment corresponding to a given combination of time of day
and traffic conditions,
said predetermined criteria including the criterion that said probe
vehicle's speed is an abnormal speed not falling within said
bandwidth.
13. A method as claimed in claim 1, further comprising:
sensing a condition in addition to the data from which probe
vehicle speed can be determined,
said predetermined criteria including the criterion that said
condition is inconsistent with a given pattern.
14. A method as claimed in claim 1, wherein estimated traffic flow
is based on the transmitted subsegment information and predictions
for a given type of day.
15. A method of estimating quantitive pattern data describing the
flow of traffic, comprising the steps of:
a) providing a plurality of calibrant vehicles,
b) providing each calibrant vehicle with respective means for
acquiring data from which speed of the calibrant vehicle at
different times and locations can be determined; and for
transmitting the acquired data to a receiving station,
c) providing at least one receiving station having means for
receiving said data transmitted by respective calibrant
vehicles,
d) at spaced times approximately equal to predetermined times of a
respective day, dispatching a respective calibrant vehicle for
operation over a substantially predetermined route,
e) during at least the portion of the day that each respective
vehicle is being operated over said route, controlling said
respective vehicle to record said data,
f) transmitting the recorded data to said at least one receiving
station,
g) computing subsegment speed samples for each calibrant vehicle
from which said data have been received, and determining baseline
data having a time-varying bandwidth descriptive of traffic
conditions on respective segments of said route for at least one
combination of time of day and traffic conditions.
16. A method as claimed in claim 15, characterized in that step b)
comprises providing each calibrant vehicle with respective means
for determining the location of the respective vehicle at
respective instants of time separated by intervals of approximately
a given period of time, for determining the time of each said
respective instant, and for recording data corresponding to the
determined location and said time for each respective instant; and
respective means for transmitting the recorded data.
17. A method as claimed in claim 16, characterized in that each
calibrant vehicle records and stores data for each of said instants
of time while being operated over at least a segment of the entire
predetermined route, prior to transmitting the stored data to said
receiving station.
18. A method as claimed in claim 16, characterized in that each
calibrant vehicle records and stores data for each of said instants
of time while being operated over the entire predetermined route,
prior to transmitting the stored data to said receiving
station.
19. A method of estimating quantitive data describing the flow of
traffic along a route, comprising the steps of:
a) determining baseline data having a time-varying bandwidth
descriptive of traffic conditions on respective segments of said
route for at least one combination of time of day and traffic
conditions,
b) analyzing said baseline data to determine the relationship
between the number of probe vehicles and the reliability of traffic
flow estimation based thereon, and selecting a first number of
probe vehicles whose reporting will provide a given reliability of
traffic flow estimation,
c) deploying a plurality of probe vehicles at respective times
approximating the time of day and traffic conditions corresponding
to said at least one combination,
d) causing each deployed probe vehicle to acquire data from which
subsegment information including the speed of that probe vehicle at
different times and locations can be determined, to compare
subsegment speed with said baseline data having a time-varying
bandwidth descriptive of traffic conditions on the segments of said
route in which the latest location lies, and to determine whether
that subsegment speed is a normal value falling within said
bandwidth for the combination of time of day, segment and traffic
conditions,
e) responsive to determination that a given probe vehicle's
subsegment speed is an abnormal speed not falling within said
bandwidth, controlling said means for transmitting in said given
probe vehicle to transmit information related to the computed
subsegment speed, and
f) computing estimated traffic flow along at least one segment of
said route based at least in part on the transmitted
information.
20. A method as claimed in claim 19, characterized in that step d)
comprises determining the location of the respective vehicle at
respective instants of time separated by intervals of approximately
a given period of time; determining the time of each said
respective instant; computing average subsegment speed between the
most recent determination of location and the previous
determination for that probe vehicle; comparing said average
subsegment speed with said baseline data having a time-varying
bandwidth descriptive of traffic conditions on the segments of said
route in which the latest location lies; and determining whether
that average subsegment speed is a normal value falling within said
bandwidth for the combination of time of day, segment and traffic
conditions.
21. A method as claimed in claim 19, wherein a multiplicity of
probe vehicles are provided, each probe vehicle being operated at
the discretion of the respective vehicle operator, further
comprising the steps of
transmitting an identification signal from a given probe vehicle
when it is placed into operation on a said route,
upon receipt of said identification signal by said one receiving
station, determining if said given probe vehicle is within
operational range,
determining if the number of probe vehicles already communicating
on routes within operational range of said one receiving station is
less than said first number, and
upon determination that said number of probe vehicles already
communicating is less than said first number, transmitting control
signals to said given probe vehicle to cause at least one further
transmission from said given probe vehicle.
22. A probe vehicle for estimating quantitive data describing the
flow of traffic along a route, comprising:
a) means for receiving and storing baseline data having a
time-varying bandwidth descriptive of traffic conditions on
respective segments of said route for at least one combination of
time of day and traffic conditions,
b) means for determining if said probe vehicle is being operated
along said route at a time approximating the time of day and
traffic conditions corresponding to said at least one
combination,
c) means for acquiring data from which subsegment information
including the speed of that probe vehicle at different times and
locations can be determined, for comparing subsegment speed with
said baseline data having a time-varying bandwidth descriptive of
traffic conditions on the segments of said route in which the
latest location lies, and for determining whether that subsegment
speed is a normal value falling within said bandwidth for the
combination of time of day, segment and traffic conditions,
d) means, responsive to determination that said probe vehicle's
subsegment speed is an abnormal speed not falling within said
bandwidth, for controlling said means for transmitting in said
given probe vehicle to transmit information related to the computed
subsegment speed.
23. A vehicle as claimed in claim 22, characterized in that step d)
comprises determining the location of the respective vehicle at
respective instants of time separated by intervals of approximately
a given period of time; determining the time of each said
respective instant; computing average subsegment speed between the
most recent determination of location and the previous
determination for that probe vehicle; comparing said average
subsegment speed with said baseline data having a time-varying
bandwidth descriptive of traffic conditions on the segments of said
route in which the latest location lies; and determining whether
that average subsegment speed is a normal value falling within said
bandwidth for the combination of time of day, segment and traffic
conditions.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of monitoring movement of
traffic along predetermined routes, where individual moving
elements can move with a high degree of discretion as to speed
except when congestion, accident or the like limit speeds. In
particular, the invention is applicable to monitoring the flow of
motor vehicles along urban or suburban roads and highways which are
subject to delays of sufficient frequency and severity that
corrective action or dissemination of information announcing a
delay are economically desirable.
The principle of the invention is applicable to any situation in
which movement is primarily limited to forward progress along a
defined path or guideway, or transfer at intersections with other
defined paths or guideways, and where there are limitations on the
possibility of dodging around slowly moving or stopped elements.
Thus, as used in the following description and claims, the term
"vehicle" should be broadly interpreted and is not limited to
wheeled vehicles or objects moving on land surfaces.
Information about traffic flow, and particularly about unusual
deviations from the flow which would be "normal" or expected for
that route at that time and the general area weather conditions,
allows emergency vehicles to be dispatched to trouble spots before
specific reports of accidents or the like are available; allows
people or vehicle operators to choose alternate routes to avoid
delays; and can be invaluable for improving the accuracy of traffic
engineering studies.
2. Description of the Prior Art
Since telephone service has become widely available, volunteer
anecdotal reporting of abnormal conditions has been one of the most
important sources of information about highway traffic flow. Aerial
scanning by reporters in small planes is highly effective for the
relatively limited areas which can be viewed in any period of time,
but this is quite expensive and becomes inoperative when weather
conditions make it most valuable. Surveillance devices such as TV
cameras can provide information on all lanes of a multi-lane
roadway at one location, but have a high unit cost, and are a
target for theft or vandalism. Further, none of the systems
described above provide outputs which are readily processed by
computers.
Direct speed measuring devices, such as Doppler radar, are quite
expensive. While they can readily provide outputs which can be
received and processed by computers, they may not provide accurate
data for stop-and-go traffic in a traffic jam.
Simple, low cost detectors can be used, but they do not usually
provide speed data directly. For example, inductive pick-up loops
can be installed in highway surfaces, with connections to a central
processor. Such a system is shown summarily in a brochure for
"California PATH", University of California, Bldg. 452 Richmond
Field Station, 1301 S. 46th Street, Richmond, Calif. 94804.
However, not only is it expensive to install a sufficient number of
such sensors along any one highway, communication of the sensors
with the central processor will require a great amount of cabling,
or dedication of a substantial transmission spectrum. Local
processing, to provide accurate speed data independent of the size
of or space between vehicles, may be required, thereby increasing
installation and maintenance cost considerably. Further, the
sensor/communication failure rate has been estimated to be about
20% per year. Buried sensors require disturbances in the road
surface and underlayment, and thus can be a cause of accelerated
roadway deterioration. As a result the relatively high cost of
fixed monitoring devices, and the continuing cost of communication
with each of them, preclude installing such devices at a sufficient
number of locations to provide detailed information for a large
area.
Many organizations are now involved in planning, studies and tests
of systems for improving the flow or safety of highway travel. Over
40 of these are referred to in Strategic Plan for Intelligent
Vehicle-Highway Systems in the United States, Report No.
IVHS-AMER-92-3, published by the Intelligent Vehicle-Highway
Society of America. Particular projects involving collection of
traffic flow information include PATH (referred to above),
GUIDESTAR (Minneapolis, Minn.), TRAVTEK (Orlando, Fla.; already
completed) and ADVANCE (Chicago, Ill.). However, none of these have
proposed a system for accurate deviation-oriented data collection
and dissemination which can minimize the required volume of
communications on a day-to-day basis.
Partly because of the high installation costs which would accompany
the systems proposed to date, the highway traveler today seldom
sees any example of high-technology traveler information systems.
Recently, major highways in many areas have signs urging motorists
to report accidents via cellular telephones; this method of
collecting information avoids high costs of installing equipment
which will be little utilized, and can provide coverage of almost
every significant event. However, it suffers the problem that some
problems are reported by too many people, thereby tying up
communications channels and the dispatchers who receive the
information; some problems are not reported at all; and anecdotal
reporting is subject to severe quantitative inaccuracy because of
subjective interpretation and the fact that drivers are too
involved with driving their vehicles to note average speeds or the
location with sufficient accuracy.
SUMMARY OF THE INVENTION
According to the invention, a system for accurate, automatic
deviation oriented monitoring of traffic flow involves deploying
calibrant vehicles for collecting and reporting detailed
information which describes vehicle speeds actually being
experienced along the routes of interest; and loading all this
information into a central station computer, where the data are
processed statistically to yield mean values, variances, mean and
standard deviation of bandwidths and mean and standard deviation of
speeds as a function of time of day, segment location, category of
day, weather, and common but irregularly occurring events which are
reported to the system by other information channels. The computer
output forms baseline data against which observations at a
particular time, category, weather, event and location can be
compared, to identify the existence of abnormal conditions, and to
quantify the abnormality.
The baseline data may then be used for multiple purposes: for
example, the mean and standard deviation of bandwidth are used to
determine the dispatch interval of probe vehicles required to
achieve a given statistical accuracy of traffic data (this
determines the minimum number of vehicles which should be equipped
to report conditions during the regular monitoring phase); and mean
and standard deviation of speed are used to program probe vehicles,
which are operated on the highways (or paths or guideways) and
measure conditions on a regular basis, so that the probe vehicles
report only unusual conditions (probe speed out of allowed
deviation from the mean). A dispatcher and/or similar central
computer may select and control the rate of reporting as a function
of time and location along segments of the routes being
monitored.
Because the inventive system does not require installation of any
hardware in or along any roads or other pathways along which
vehicle flow is to be monitored, the system can be deployed
quickly. Further, once the equipment for calibrant vehicles (and/or
probe vehicles) and central processing has been acquired, the
monitoring system can readily be expanded to cover additional
routes. Monitoring can be transferred to a substitute route in the
event, for example, of unexpected closing of a major route because
of a catastrophe.
In a preferred embodiment, most or all of the probe vehicles are
motor vehicles which are expected to be routinely traveling the
desired roadway route segments while conducting normal other
business. Each vehicle is equipped with a differential Global
Positioning System (GPS) receiver, a small computer, and a cellular
phone or other mobile transceiver for reporting to one of a number
of receiving stations. Operation is fully automatic, the on-board
system being linked to the ignition system and/or transmission
controls, so that it reports only when it is being driven. This
embodiment involves the lowest possible long term operating costs,
because no or only a few probe vehicle communications are
required.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a diagram of a system according to the invention while
data are being collected in the calibration stage,
FIG. 2 is a diagram of a system configured for routine reporting of
abnormal conditions during the monitoring phase,
FIG. 3 is a graph of the distribution of speeds which may be
observed for a particular segment of a route,
FIG. 4 is a graph of the ratio of energy in a given bandwidth to
the energy in the entire speed signal for the segment of FIG. 3,
and
FIG. 5 is a graph showing a time varying bandwidth for the route
segment.
DESCRIPTION OF THE PREFERRED EMBODIMENT
A total system operated according to the invention includes
equipment shown diagrammatically in FIG. 1 during the calibration
phase, and equipment shown diagrammatically in FIG. 2 during the
monitoring phase.
Calibration Phase
During the calibration phase, a substantial number of calibrant
vehicles 10 will be deployed. Factors involved in selecting this
number will be described below. Each calibrant vehicle 10 is
equipped with a location sensing system, such as a GPS receiver 12.
A GPS antenna 13 is mounted in a convenient location on or near the
vehicle roof. For monitoring traffic on closely spaced roadways, it
is desirable to obtain position information accurate to
approximately one meter; for example, 0.5 meter. This permits
distinguishing lane changes, and the particular lane of a
multi-lane roadway being travelled. The time of each position
reading must also be recorded, but this is readily available in
most computers (high relative accuracy) and from GPS receivers
(high absolute accuracy).
Because military security considerations have caused governmental
agencies to add noise to the transmitted GPS signals, the
commercial GPS systems produce location data accurate to only
perhaps 30 meters. However, a GPS receiver operated at a known,
fixed location can be used to provide a differential correcting
signal, which is then transmitted to a differential receiver, for
example over an FM sub-carrier to another antenna 15 connected to a
special FM receiver 16 in the vehicle. The receiver 16 then
communicates the differential information to the GPS. Of course,
the differential signal receiver and GPS unit can be integrated
into one box.
A computer 18, such as a laptop computer, is installed in the
vehicle 10. This computer has data inputs from the GPS receiver 12
and from the vehicle ignition or control system 20. Position
readings are taken, and the time and position is stored,
frequently; for example, every 5 seconds. Position readings may be
recorded as latitude and longitude. Although the GPS system may
provide a direct velocity output value, it will usually be
undesirable to use this reading because it reflects an average
calculated for a time period which may not reflect traffic flow as
being modeled. For terrestrial highway travel, any altitude data
which may be available will usually be ignored. The total number of
readings in a nominal 8-hour day is then between 5000 and 6000, so
that storage capacity is not a problem even with a small laptop
computer.
A cellular phone 22 may optionally be included. This provides an
opportunity for driver communication with a dispatcher at a central
station. However, this phone will not ordinarily be used for
frequent reporting. Instead, to reduce communication cost during
the calibration period, data may be transferred by storing it on a
floppy disc which is periodically carried to the computer 40.
Alternatively, for transmitting stored data to a modem 30 which is
then functioning as the communications port for a data receiving
station, the vehicle operator may establish a connection from the
laptop computer (via a modem not shown) to the vehicle phone, or
may carry the laptop to a telephone at home or office to transmit
the data via the telephone network 31 and modem 30 to a central
computer 40 for compiling and statistically evaluating the data
collected from all the calibrant vehicles 10.
The calibration phase will involve, for each route to be monitored,
a number of days sufficient to provide a minimum level of
confidence in the resulting estimates, such as four weeks during
each season. The number of calibrant vehicles involves a trade-off
between minimizing the number of weeks or months required to obtain
statistically significant data and the cost of vehicle leases,
equipment purchase or lease, and driver selection and training.
Where the routes of interest are relatively long or slow, an
individual calibrant vehicle may be able to make only one useful
one-way trip during the peak traffic period. Another factor to be
considered is traffic diversion to alternate routes, resulting from
drivers' reactions to existing radio reports of conditions or
reactions to perceived patterns of the recent past. Thus on a given
day it may be desirable to provide at least some coverage on
selected routes which are generally parallel to a route which is
receiving full calibration coverage.
An initial decision must be made as to the number of routes to be
covered simultaneously, and the degree to which "fine-grain"
analysis is to be provided for any route. There is an obvious
choice between deploying a larger fleet of calibrant vehicles, so
as to cover a greater number of routes during a given period of
time, thereby completing the entire calibration phase sooner; and a
lower initial investment in equipment and personnel by using a
sufficient fleet to cover a smaller number of routes
simultaneously, and stretching the calibration phase over a greater
number of months. A pattern equivalent to 20 days (5 days per week,
for 4 weeks) of full coverage per route is suggested.
Because of long-term effects like highway construction, climatic
variation over the course of a year, or anticipated seasonal or
special-occasion variations in traffic volume, on any given route
the calibration days or weeks may not be planned for successive
days or weeks. Where extensive interleaving of coverage days for
various routes is used, computer analysis of the data may uncover
correlations between the data patterns which are not readily
recognized by a human, and therefore can improve the accuracy both
of modeling and of subsequent reporting or prediction based on
probe data during the monitoring phase.
The dispatching/data recording protocol during calibration may, for
example, call for dispatching another calibration vehicle every 5
to 15 minutes during rush hour or other busy times. While the
calibration system is in an operating mode, for example while the
ignition is turned on, at the predetermined intervals of time (at
least every 15 seconds, and preferably every 5 seconds or more
often) the latitude, longitude and time are recorded by the
computer 18. To minimize use of radio or telephone transmission
channel space and expense, as described above, during calibration
the computer will store all the data for one or more trips, or for
a half-day or day's travel or even longer. The information is
stored on, or copied onto, a floppy disc which is physically
delivered to the central computer; or, if the distances involved
are substantial, delivered to a computer receiving station for
transmission over a computer network or a telephone line. Typical
floppy discs can store about 2 months of data stored continuously
at 5 second intervals.
In order to improve the accuracy of the models constructed from the
calibration data, it may also be desirable to record other data
available automatically at the calibrant vehicle. For example,
operation of the windshield wipers for more than a windshield
washer interval indicates precipitation. If an electronic sensor
monitors outside temperature, this can be used to determine whether
it is probably rain or something worse. If the wipers are operating
in an intermittent mode, the rain is not heavy; while if they are
operating at highest speed, rain is probably heavy. Depending on
laws and driver training, operation of the headlights may indicate
darkness; otherwise, a photo sensor may advantageously provide data
to be recorded, whether it is bright, heavily overcast, or
dark.
Modeling
A special feature of the invention is the use made of the raw
calibration data. The essential quantity of interest is vehicle
speed. However, physical constraints place limits on the time
variations of the speed, which implies that the spectrum of the
speed signals is limited. Thus these signals may be viewed as a
Band-limited Stochastic Process.
Because the spectrum and bandwidth of the speed signals normally
change slowly, in a given interval of time they will have a
constant mean and variance. This "given interval" is specific to
the time of day, and is determined by evaluation of the data taken
during calibration. If v(s,t) is the speed, at time t, of a vehicle
starting at time s, s is then the start of a length of travel which
may overlap several segments. Because of the restraints always
affecting vehicle travel, v(s,t) is essentially band-limited for
each s. The spectrum V(s,f) of v(s,t) then reflects the frequency
content of v(s,t). The graph of FIG. 3 shows the Fourier transform
of the speed along a segment. This produces the distribution
.vertline.V(s,f).vertline. for a fixed s.
To determine what is a "normal" variation from the mean, the graph
of FIG. 4 shows the ratio B(s,w) of the energy in the bandwidth
from 0 (zero) to w, to the entire energy, as a function of the
bandwidth w. More simply put, it is the area under the curve of
FIG. 3 that is included by setting limits between 0 (zero) and a
fixed frequency w divided by the total area. In this context,
energy is defined as the integral of the square of the absolute
value of the Fourier Transform of the speed signals and is the full
area under the curve of FIG. 3. It is given by the equation
##EQU1## Assuming that a value B(s,W(s))=0.95 is a good compromise
between cost of extensive reporting, and ineffective monitoring, a
sampling time or Nyquist rate would be T(s)=1/(2W(s)). Assuming a
slow variation of T(s) over a suitable interval of time, T(s) may
be used as the time interval for dispatch or selection of probe
vehicles during the monitoring phase. The Nyquist-Shannon theorem
can then be used to reconstruct v(s,t) from the samples {v(c,T(s),
v(s,2T(s)), . . . , } transmitted by the probe vehicle during the
monitoring phase.
The data collected for a given route segment during the calibration
period may be evaluated by providing a "graph" showing the mean and
the variance of bandwidth as a function of coarse time and
location; but it is likely that a weather axis, a holiday axis, or
others may also be employed. The velocity patterns of days with
different characteristics may be essentially the same; in that case
one pattern should be used for both. Other pattern relationships
may also be discernible; for example, one or a succession of
below-average-speed days on a given route may frequently be
followed by an above-average speed day because motorists tend to
change their route selection because of the immediately prior bad
travel days. In such a situation the standard for reporting
"abnormal" conditions would be altered for the anticipated
above-average-speed day.
By comparing the model produced if data from less than all of the
calibrant vehicles are used, the degradation of accuracy with
reduction in number of reporting vehicles can be determined. This
can be used to improve the cost-accuracy trade-off during later
sequences of the calibration stage, as well as during the
monitoring phase.
Monitoring Phase
On-line monitoring and reporting activity can start more-or-less as
soon as the calibration phase is completed. To give the exact
number and frequency of deployment for a given route segment, the
bandwidth of an origin-destination pair directly gives the probe
coverage needed for a given accuracy.
The equipment used for this phase, shown in FIG. 2, preferably
differs substantially in numbers, and somewhat in kind, from that
used for calibration. Each probe vehicle 110 has a GPS receiver 12
and antenna 13, a differential data receiver 16 and its antenna 15,
and a cellular phone 22 with antenna 23 which may be identical to
those previously used in a calibrant vehicle. However, the probe
computer 118 is provided (or down-loaded by telephone/modem
communication) with a stored record of bandwidth patterns for one
or all of the routes, and is programmed and connected to transmit
its speed data automatically over the cellular phone 22 whenever
the measured bandwidth differs from the mean bandwidth obtained
from the calibration phase by a programmed amount. The bandwidth is
measured in real time as the probe travels over each segment.
Pattern selection can be fully automatic when the day is "normal"
for that route. As is now commonplace, the computer 118 has an
internal clock and calendar. Holiday and major special events are
known so far in advance that they will be part of the programmed
data which are provided on a periodic basis, preferably by mailing
up-date data on floppy discs or the equivalent. Even routes which
are affected by major sporting events will have patterns
established, during the calibration period, which take into account
the impact on traffic flow. Each day is expected to follow one of
the patterns of mean and standard deviation of speed, as a function
of time and location, which is predicted for that type of day.
Observed speed data are stored in the computer 118 only to gather
data which indicates a specific mean and variance for the current
segment (location). Any speed outside the acceptable variation will
cause the probe system to call, via the commercial telephone
network including a transceiver 130, to a central computer 140.
The central computer 140 is programmed to provide information on
speed; or more significantly, on places where speed is outside
normal speeds, via a display 142. Additionally, the computer will
automatically activate selected probe vehicles, by messages
transmitted over the cellular telephone network, in order to have
sufficient number of active probes in each significant segment of a
route. Further, if the computer is unable to activate sufficient
probe vehicles, it will provide an alarm and specific information
over the display 142, so that a dispatcher can take specific
action, which might include dispatching one or more special probe
vehicles.
Activation of a probe vehicle presupposes that one is available.
During the monitoring phase, in a system according to the invention
a relatively large number of vehicles will be equipped so that they
can serve as probe vehicles. Desirably, these vehicles are selected
because they will normally or frequently be operating on routes of
interest at times of interest, independent of their status as probe
vehicles. Examples might be commuter buses, delivery vehicles, or
private automobiles frequently used for commuting. These vehicles
will be equipped as probes 110. In one preferred mode of operation,
upon entering any route which is normally monitored, the probe
computer 118 will automatically seek to communicate, via the phone
22 and any transceiver 130 within operational range, with the
central computer 140 to register as available for activation. The
computer will then reply, confirming the contact, and directing
activation or directing that this probe not communicate
further.
In another mode of operation, using essentially the same equipment,
the transceivers 22 and 130 are not operated as part of a general
purpose cellular telephone system, but use one or more channels or
time slots of a mobile radio system. The receiving stations can be
satellite transceivers, or cellular spaced transceivers having
restricted service channels or time slots. In this mode, for
example, the central computer 140 may select a particular cellular
transceiver whose operational range covers a route segment for
which data are desired, and transmit a coded request for probes,
which are within range and are on that route segment, to reply. Any
of the well known techniques for preventing or reducing collisions
between replying transceivers 22 may be implemented. If too many
probes reply, the computer will select those to activate, and those
to refrain from automatic transmission of variance data.
According to another aspect of the invention, during the monitoring
phase the computer will transmit, to one or to all probes
listening, control information for changing the speed and variance
for one or more route segments, where information from probe
vehicles or from outside sources suggest that a different pattern
is to be expected. A common example of this situation is area-wide
inclement weather, or weather which is expected to affect or is now
affecting one route or region. The change can either be a specific
quantitative change, or can be directing use of a different stored
pattern.
Another trigger to substitution of alternative patterns is on-board
sensing. For example, continuous operation of windshield wipers, if
sensed, may cause the computer to switch automatically to a "rainy
day" pattern; however, if an on-board thermometer senses an
exterior temperature which is close to or below freezing, a
snow/ice pattern may be substituted. Following the principle that
data are transmitted only when there is a deviation from the
expected pattern, some or all probe vehicles may be equipped to
sense temperature, wiper operation, or brightness/darkness, and to
transmit a "conditions deviation" signal if this condition is not
consistent with the pattern which had been in use. Dead reckoning
can be used to supplement GPS when the terrain (for example,
tunnels or tall buildings) blocks GPS reception.
In another operating variation, the central computer 140 can infer
the current state of traffic flow by recording the last car that
"calls in" as the valid speed. This information should, in turn, be
transmitted to later probes so that when traffic returns to
"normal" a call is received to that effect. Such a mode is
particularly useful if a vehicle breakdown or minor accident has
created a very abnormal flow, which is corrected by people at the
scene without the knowledge of or any action by police, tow trucks,
or the like.
A further aspect of the invention is automatic up-dating. Even
though the number of vehicles used as probes will normally be
smaller than that used as calibrant vehicles, changes in the
bandwidth, noted as a pattern of variances, can automatically be
used to adjust the pattern model for the type of day or route. Only
when a major permanent change occurs suddenly, such as the opening
of an additional highway, is there reason to provide a new
calibration phase.
Dissemination of information obtained from practice of the
invention can be by any well-known technique. Some highways already
have low-power transmitters, operating in channels of the radio
broadcasting bands, for local traffic or other information. Message
up-dates can be provided on these transmitters directly under
control of the computer in the central station; or can be directed
by a system dispatcher. The display 142 can use automatically
presented maps on a monitor or a board, with color or number
indications of trouble spots; or can include a plain text message
describing variance information, and indicating possible
explanations for this variation based on similarity of the
variation to some stored pattern of past recurring or unique
occurrences.
When a probe vehicle is operating on a route which has no
calibration data, reporting would ordinarily be suppressed.
However, a driver-operable override can be provided, to cause the
on-board transceiver to attempt to communicate automatically when
the driver believes that the situation is abnormal and deserves
reporting. In this situation, the extreme accuracy of the GPS
location signal allows the central computer 140 to determine that
the location reported is in fact a driving lane of a roadway; and
exactly where and what the speed pattern is. This permits not only
dissemination of traffic information about such roads, but also may
pinpoint a condition requiring investigation by police.
A further variation of the above operating mode permits automatic
attempted override reporting whenever the on-board system
identifies an extended period of limited or no movement while on a
route of interest. Normally such a situation is the result of an
accident or the like where locating the cause may be difficult
unless aerial observation is possible. The automatically reported
data, if accepted by the computer, can provide valuable
identification of the extent or location of a serious abnormality,
long before other normally activated probes may start sending data.
Furthermore, since the accuracy permits distinguishing between
points on a driving lane and points on a highway shoulder, and the
duration of the occurrence, the authenticity of the automated
reporting makes the report credible.
Although the system may be operating nominally in the monitoring
phase, it is possible to continue to refine calibration during day
to day operations by using the probe fleet in the calibration mode.
Further, if a probe vehicle is operated off the normal paths or
terrain, it may be desirable to include data on that route for the
database.
Other embodiments
The Global Positioning System is described as the source of
location information because it is the best system now known for
obtaining position information, with sufficient accuracy, that is
fully automatic, provides results easily processed by computers,
and does not require special installations along a path or roadway.
However, it is clear that many other methods of providing position
information are possible and may become available or be installed
in the near future. During the calibration phase it may be possible
to acquire data from which location as a function of time can be
determined through use of an on-board inertial navigation system.
Such a system might be too expensive for installation in probe
vehicles, but would not suffer the disadvantage of signal blocking
in tunnels or in relatively narrow roadways between tall buildings.
During the calibration or monitoring phases, "dead reckoning" data
may be supplemented by sensing location identifier signals
transmitted at checkpoints from a coil or a small directional
antenna. For example, vehicle speed can be sensed accurately by a
wheel speed sensor and, when integrated with vehicle steering
angles, can provide fairly accurate dead reckoning position
information for the distance between checkpoints.
The cellular phone 22 may also be used for direct communication
between the vehicle driver and personnel at the computer station,
to report extraordinary occurrences, so that they may be considered
in the overall evaluation, or may be used to alter instructions
which may be given over that same phone to the vehicle
operator.
When applied to other situations besides motor vehicles on a
roadway, the invention merely requires that calibrants be able to
acquire data from which accurate time and location information can
be determined, and have respective means for storing and
transmitting the information during a calibration phase. During
monitoring a sufficient number of probes must be available, each
having access to data from which time, location and speed can be
determined, computing capability for storing patterns of speed and
bandwidth, and equipment for transmitting data relating to
out-of-band conditions to a receiving station so that evaluation of
individual reports and corrective action, warnings, or the like are
possible. Thus the invention could even be applied to movement of
people on foot in a large terminal or building complex having
well-defined corridors and stairwells. In this situation altitude
data, or some other indication of the floor level or particular
flight in a stack of stairs, will usually be required in addition
to position on a surface.
* * * * *